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Comparison of the Bottom Nepheloid Layer and Late Holocene Deposition on Nitinat Fan: Implications for Lutite Dispersal and Deposition

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Comparison of the Bottom Nepheloid Layer and Late Holocene Deposition on Nitinat Fan: Implications for Lutite Dispersal and Deposition Comparison of the bottom nepheloid layer and late Holocene deposition on Nitinat Fan: Implications for lutite dispersal and deposition PER R. STOKKE* I Department of Geological Sciences, Center for Marine and Environmental Studies, Lehigh University, BOBB CARSON J Bethlehem, Pennsylvania 18015 EDWARD T. BAKER* Department of Oceanography, University of Washington, Seattle, Washington 98195 ABSTRACT Suggested emplacement mechanisms for these lutites include ac- cumulation from low-density turbidity flows (Moore, 1967; A study of 56 sediment cores and 121 nephelometer profiles from Shepard and others, 1969; Piper, 1970; Nelson and Kulm, 1973), Nitinat deep-sea fan shows variations in late Holocene accumula- particle-by-particle (hemipelagic) deposition from the overlying tion rates and sediment texture which parallel variations in thick- water column (Huang and Goodell, 1970; Piper, 1970; Lisitzin, ness and suspended sediment load of the bottom nepheloid layer. 1972; Nelson and Kulm, 1973; Bouma and Hollister, 1973), or re- Furthermore, accumulation rates, sediment texture, and nepheloid deposition of winnowed sediments (Piper, 1970; Huang and layer variables all show a substantial degree of correlation with fan Goodell, 1970; Lisitzin, 1972; Bouma and Hollister, 1973; Biscaye topography. In general, the nepheloid layer thickens (>100 m) and and Eittreim, 1974). To date, no preference can be give:n to any one suspended sediment loads increase (>100 /ug/cm2) above Cascadia of these processes on modern deep-sea fans, nor is it clear what role Channel (the major channel crossing the fan) as well as above the the bottom nepheloid layer plays in the depositional process. northern flank of the fan. Over levees and the western portion of In part, the problem stems from our ignorance of the relationship the fan, the nepheloid layer thins to <50 m, and suspended sedi- between suspended matter and lutite accumulation. To assess the 2 ment loads fall below 100 /xg/cm . Cascadia Channel and the contribution of modern suspended sediments to a deep-sea fan, this northern flank of the fan have been loci of rapid sedimentation, study relates late Holocene accumulation rates, sediment texture, with accumulation rates ranging from approximately 5 to greater topography, and thickness and intensity of the nepheloid layer on 2 than 12 mg/cm /yr. In contrast, the apex and western reaches of the Nitinat Fan. While the results do not uniquely define a lutite 2 fan have significantly lower accumulation rates: 1 to 5 mg/cm /yr. emplacement mechanism, they clearly constrain the models Detailed size analysis of bottom sediments shows that areas charac- suggested. terized by rapid sedimentation and a thick, heavily loaded nepheloid layer have more medium to fine silt (5 to 7 cj>) than the TOPOGRAPHY clay-rich (>8 (f>) sediments from interchannel areas with low ac- cumulation rates and a thin, lightly loaded nepheloid layer. The Nitinat deep-sea fan is located in northern Cascadia Basin, with data suggest that turbid water moves continuously down Cascadia its apex at lat 47°55'N, long 126°30'W. It lies at the base of the Channel and the northern flank. The transport mechanism is size- continental slope off Washington and Vancouver Island, west of selective and topographically controlled, concentrating silt-sized Nitinat, Juan de Fuca, and Barkley submarine canyons (Fig. 1), detritus in topographic lows. The data also suggest a positive which apparently feed onto it. Cascadia Channel, the dominant val- downward flux of sediment particles within the nepheloid layer, at ley on the fan, issues from the joint terminus of Nitinat and Juan de least when averaged over a significant period of time. Fuca Canyons and trends southward along the margin of the slope. The channel widens just south of the apex to form a series of sub- INTRODUCTION channels, before assuming a distinct V-shape farther down channel (at lat 47°38'N). To the west and north, the channel is bordered by Deep-sea fans are major sedimentary features off some continen- a well-developed natural levee with a relief of 50 to 75 m (at depths tal margins. As such, they have been the subject of numerous less than 2,400 m; Fig. 1). studies (Gorsline and Emery', 1959; Shepard and others, 1969; In contrast, the northern flank of the fan is relatively featureless Normark, 1970; Piper, 1970; Curray and Moore, 1971; Normark and has no apparent connection to the canyons egressing at the and Piper, 1972; Hein, 1973; and others) describing bathymetric apex. Barkley Canyon appears to open onto the northern flank, but development, dispersal patterns, and rates of accumulation. These no associated channel is observed on the fan. investigations have emphasized turbidity currents as the primary The southwestern portion of the fan displays a complex topog- mechanism of transportation and deposition of sand and sandy silt. raphy with several small, discontinuous valleys. Some of these cross In contrast, relatively little attention has been paid to the the fan and feed into Cascadia Channel south of lat 46°30'N. processes controlling dispersal and accumulation of fine-grained Others can be traced only a few kilometres and are, apparently, sediments (clayey silt and clay) on deep-sea fans. Whereas some remnants of channel migration. material is undoubtedly transported by turbidity currents, a sig- nificant portion shows no evidence of turbidite deposition, particu- METHODS larly in sediments deposited during the past 9,000 to 12,000 yr (Shepard and others, 1969; Piper, 1970; Carlson and Nelson, Data and samples for this study were collected during three 1969; Huang and Goodell, 1970). cruises from 1971 to 1974. During these cruises, 121 nephelometer 5 Present addresses: (Stokke) Continental Shelf Institute, Hakon Mag- profiles were recorded in the waters over the fan. The integrating nussonsgt. IB, 7000 Trondheim, Norway. (Baker) Pacific Marine Environ- nephelometer used in the study was described by Sternberg and mental Laboratory, NOAA, 3711 15th Avenue NE, Seattle, Washington others (1974). 98105. Recognition of the precise top of the bottom nepheloid layer is Geological Society of America Bulletin, v. 88, p. 1586-1592, 12 figs., November 1977, Doc. no. 71106. 1586 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/11/1586/3429189/i0016-7606-88-11-1586.pdf by guest on 27 September 2021 BOTTOM NEPHELOID LAYER AND LATE HOLOCENE DEPOSITION ON NITINAT FAN 1587 127° 30' 127° 00' 126° 30' 126° 00' 127° 30' 127" 00' 126° 30' 126*00' Figure 1. Study area in the northeast Pacific Ocean, bathymetry of Nitinat Fan, and locations of the cross sections in Figure 5. somewhat subjective. In this study, the nepheloid layer is opera- RESULTS tionally defined as the zone of increasing light-scattering extending from the deepest depth of clearest water to the sea floor. The scat- Nepheloid Layer tering intensity within this layer can be related to the mass of sedi- ment suspended in the nepheloid layer per unit cross section of the Figures 2 and 3 are composite maps of the nepheloid layer thick- water column, a measure obtained by calibration of the relation of ness and suspended sediment load from three cruises (1971 to the scattering intensity and suspensate concentration and integra- 1974). The primary characteristics of the nepheloid layer remained tion of the appropriate area under the nephelometer profile. stable over this period, and, hence, composite maps can reasonably The study of bottom sediments is based on 56 gravity cores. Ac- be drawn (Baker and others, 1974; Baker, 1976). cumulation rates (in mg/cm2/yr) were calculated according to the The thickness of the nepheloid layer (Fig. 2) is in general con- method of Koczy (1951). Results are based on a previously defined trolled by the topography of the fan, since the regional trend of the stratigraphy and an average sediment density of 2.64 g/cm3. top of the nepheloid layer is a gradual seaward deepening. The The time stratigraphy used is based on faunal analysis (plankton nepheloid layer is thickest (>100 m) over both Cascadia Channel formainifera/radiolarian ratio) and 16 radiocarbon dates (Carson, and the steeply dipping northwestern flank of the fan. Within Cas- 1971). Stratigraphic units (early Holocene, 13,000 to 9000 B.P.; cadia Channel, the nepheloid layer gradually thickens downchan- late Holocene, 9000 B.P. to present) were defined by a modified ac- nel, culminating in an abrupt increase (to >200 m) where the ceptance sampling procedure.(Carson and McManus, 1971). channel becomes sharply constricted (near lat 47°38'N). Over the Size analyses (0 <f> to >11 <f> in whole <f> intervals) were run onorthwesn t flank, the thickness of the nepheloid layer rapidly in- material from the top 5 cm in each core. Sediment coarser than 4 (f> creases away from the levee crest and then decreases again as the was dry sieved; finer material was analyzed by pipetting in a sea-floor slope becomes gentler. This region of thickness >100 m temperature-controlled bath. extends around the corner of the levee as far south as approxi- Textural variations were examined by subjecting all size dis- mately lat 47°45'N. The nepheloid layer thins to <50 m over the tributions to an R-mode factor analysis, using the weight percent- levee crest west and north of Cascadia Channel, over the base of the age in each size class as variables. The factor analysis gives factor continental slope (which defines the channel's eastern edge), and measures as output. The factor measures were used as input to a over the broad southwestern portion of the fan. Q-mode cluster analysis using a distance function as the classifying The pattern of suspended sediment loads in the nepheloid layer criteria. This sequence of programs was modified from Parks (Fig. 3) has a general configuration similar to its thickness.
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